// Package peers provides information about peers at the Ethereum protocol level.
// "Protocol level" is the level above the network level, so this layer never sees or interacts with (for example) hosts that are
// uncontactable due to being down, firewalled, etc.  Instead, this works with peers that are contactable but may or may not be of
// the correct fork version, not currently required due to the number of current connections, etc.
//
// A peer can have one of a number of states:
//
// - connected if we are able to talk to the remote peer
// - connecting if we are attempting to be able to talk to the remote peer
// - disconnecting if we are attempting to stop being able to talk to the remote peer
// - disconnected if we are not able to talk to the remote peer
//
// For convenience, there are two aggregate states expressed in functions:
//
// - active if we are connecting or connected
// - inactive if we are disconnecting or disconnected
//
// Peer information is persistent for the run of the service.  This allows for collection of useful long-term statistics such as
// number of bad responses obtained from the peer, giving the basis for decisions to not talk to known-bad peers.
package peers

import (
	"errors"
	"sort"
	"sync"
	"time"

	"github.com/libp2p/go-libp2p-core/network"
	"github.com/libp2p/go-libp2p-core/peer"
	ma "github.com/multiformats/go-multiaddr"
	pb "github.com/prysmaticlabs/prysm/proto/beacon/p2p/v1"
	"github.com/prysmaticlabs/prysm/shared/bytesutil"
	"github.com/prysmaticlabs/prysm/shared/roughtime"
)

// PeerConnectionState is the state of the connection.
type PeerConnectionState int

const (
	// PeerDisconnected means there is no connection to the peer.
	PeerDisconnected PeerConnectionState = iota
	// PeerConnecting means there is an on-going attempt to connect to the peer.
	PeerConnecting
	// PeerConnected means the peer has an active connection.
	PeerConnected
	// PeerDisconnecting means there is an on-going attempt to disconnect from the peer.
	PeerDisconnecting
)

var (
	// ErrPeerUnknown is returned when there is an attempt to obtain data from a peer that is not known.
	ErrPeerUnknown = errors.New("peer unknown")
)

// Status is the structure holding the peer status information.
type Status struct {
	lock            sync.RWMutex
	maxBadResponses int
	status          map[peer.ID]*peerStatus
}

// peerStatus is the status of an individual peer at the protocol level.
type peerStatus struct {
	address               ma.Multiaddr
	direction             network.Direction
	peerState             PeerConnectionState
	chainState            *pb.Status
	chainStateLastUpdated time.Time
	badResponses          int
}

// NewStatus creates a new status entity.
func NewStatus(maxBadResponses int) *Status {
	return &Status{
		maxBadResponses: maxBadResponses,
		status:          make(map[peer.ID]*peerStatus),
	}
}

// MaxBadResponses returns the maximum number of bad responses a peer can provide before it is considered bad.
func (p *Status) MaxBadResponses() int {
	return p.maxBadResponses
}

// Add adds a peer.
// If a peer already exists with this ID its address and direction are updated with the supplied data.
func (p *Status) Add(pid peer.ID, address ma.Multiaddr, direction network.Direction) {
	p.lock.Lock()
	defer p.lock.Unlock()

	if status, ok := p.status[pid]; ok {
		// Peer already exists, just update its address info.
		status.address = address
		status.direction = direction
		return
	}

	p.status[pid] = &peerStatus{
		address:   address,
		direction: direction,
		// Peers start disconnected; state will be updated when the handshake process begins.
		peerState: PeerDisconnected,
	}
}

// Address returns the multiaddress of the given remote peer.
// This will error if the peer does not exist.
func (p *Status) Address(pid peer.ID) (ma.Multiaddr, error) {
	p.lock.RLock()
	defer p.lock.RUnlock()

	if status, ok := p.status[pid]; ok {
		return status.address, nil
	}
	return nil, ErrPeerUnknown
}

// Direction returns the direction of the given remote peer.
// This will error if the peer does not exist.
func (p *Status) Direction(pid peer.ID) (network.Direction, error) {
	p.lock.RLock()
	defer p.lock.RUnlock()

	if status, ok := p.status[pid]; ok {
		return status.direction, nil
	}
	return network.DirUnknown, ErrPeerUnknown
}

// SetChainState sets the chain state of the given remote peer.
func (p *Status) SetChainState(pid peer.ID, chainState *pb.Status) {
	p.lock.Lock()
	defer p.lock.Unlock()

	status := p.fetch(pid)
	status.chainState = chainState
	status.chainStateLastUpdated = roughtime.Now()
}

// ChainState gets the chain state of the given remote peer.
// This can return nil if there is no known chain state for the peer.
// This will error if the peer does not exist.
func (p *Status) ChainState(pid peer.ID) (*pb.Status, error) {
	p.lock.RLock()
	defer p.lock.RUnlock()

	if status, ok := p.status[pid]; ok {
		return status.chainState, nil
	}
	return nil, ErrPeerUnknown
}

// SetConnectionState sets the connection state of the given remote peer.
func (p *Status) SetConnectionState(pid peer.ID, state PeerConnectionState) {
	p.lock.Lock()
	defer p.lock.Unlock()

	status := p.fetch(pid)
	status.peerState = state
}

// ConnectionState gets the connection state of the given remote peer.
// This will error if the peer does not exist.
func (p *Status) ConnectionState(pid peer.ID) (PeerConnectionState, error) {
	p.lock.RLock()
	defer p.lock.RUnlock()

	if status, ok := p.status[pid]; ok {
		return status.peerState, nil
	}
	return PeerDisconnected, ErrPeerUnknown
}

// ChainStateLastUpdated gets the last time the chain state of the given remote peer was updated.
// This will error if the peer does not exist.
func (p *Status) ChainStateLastUpdated(pid peer.ID) (time.Time, error) {
	p.lock.RLock()
	defer p.lock.RUnlock()

	if status, ok := p.status[pid]; ok {
		return status.chainStateLastUpdated, nil
	}
	return roughtime.Now(), ErrPeerUnknown
}

// IncrementBadResponses increments the number of bad responses we have received from the given remote peer.
func (p *Status) IncrementBadResponses(pid peer.ID) {
	p.lock.Lock()
	defer p.lock.Unlock()

	status := p.fetch(pid)
	status.badResponses++
}

// BadResponses obtains the number of bad responses we have received from the given remote peer.
// This will error if the peer does not exist.
func (p *Status) BadResponses(pid peer.ID) (int, error) {
	p.lock.RLock()
	defer p.lock.RUnlock()

	if status, ok := p.status[pid]; ok {
		return status.badResponses, nil
	}
	return -1, ErrPeerUnknown
}

// IsBad states if the peer is to be considered bad.
// If the peer is unknown this will return `false`, which makes using this function easier than returning an error.
func (p *Status) IsBad(pid peer.ID) bool {
	p.lock.RLock()
	defer p.lock.RUnlock()

	if status, ok := p.status[pid]; ok {
		return status.badResponses >= p.maxBadResponses
	}
	return false
}

// Connecting returns the peers that are connecting.
func (p *Status) Connecting() []peer.ID {
	p.lock.RLock()
	defer p.lock.RUnlock()
	peers := make([]peer.ID, 0)
	for pid, status := range p.status {
		if status.peerState == PeerConnecting {
			peers = append(peers, pid)
		}
	}
	return peers
}

// Connected returns the peers that are connected.
func (p *Status) Connected() []peer.ID {
	p.lock.RLock()
	defer p.lock.RUnlock()
	peers := make([]peer.ID, 0)
	for pid, status := range p.status {
		if status.peerState == PeerConnected {
			peers = append(peers, pid)
		}
	}
	return peers
}

// Active returns the peers that are connecting or connected.
func (p *Status) Active() []peer.ID {
	p.lock.RLock()
	defer p.lock.RUnlock()
	peers := make([]peer.ID, 0)
	for pid, status := range p.status {
		if status.peerState == PeerConnecting || status.peerState == PeerConnected {
			peers = append(peers, pid)
		}
	}
	return peers
}

// Disconnecting returns the peers that are disconnecting.
func (p *Status) Disconnecting() []peer.ID {
	p.lock.RLock()
	defer p.lock.RUnlock()
	peers := make([]peer.ID, 0)
	for pid, status := range p.status {
		if status.peerState == PeerDisconnecting {
			peers = append(peers, pid)
		}
	}
	return peers
}

// Disconnected returns the peers that are disconnected.
func (p *Status) Disconnected() []peer.ID {
	p.lock.RLock()
	defer p.lock.RUnlock()
	peers := make([]peer.ID, 0)
	for pid, status := range p.status {
		if status.peerState == PeerDisconnected {
			peers = append(peers, pid)
		}
	}
	return peers
}

// Inactive returns the peers that are disconnecting or disconnected.
func (p *Status) Inactive() []peer.ID {
	p.lock.RLock()
	defer p.lock.RUnlock()
	peers := make([]peer.ID, 0)
	for pid, status := range p.status {
		if status.peerState == PeerDisconnecting || status.peerState == PeerDisconnected {
			peers = append(peers, pid)
		}
	}
	return peers
}

// Bad returns the peers that are bad.
func (p *Status) Bad() []peer.ID {
	p.lock.RLock()
	defer p.lock.RUnlock()
	peers := make([]peer.ID, 0)
	for pid, status := range p.status {
		if status.badResponses >= p.maxBadResponses {
			peers = append(peers, pid)
		}
	}
	return peers
}

// All returns all the peers regardless of state.
func (p *Status) All() []peer.ID {
	p.lock.RLock()
	defer p.lock.RUnlock()
	pids := make([]peer.ID, 0, len(p.status))
	for pid := range p.status {
		pids = append(pids, pid)
	}
	return pids
}

// Decay reduces the bad responses of all peers, giving reformed peers a chance to join the network.
// This can be run periodically, although note that each time it runs it does give all bad peers another chance as well to clog up
// the network with bad responses, so should not be run too frequently; once an hour would be reasonable.
func (p *Status) Decay() {
	p.lock.Lock()
	defer p.lock.Unlock()
	for _, status := range p.status {
		if status.badResponses > 0 {
			status.badResponses--
		}
	}
}

// BestFinalized returns the highest finalized epoch equal to or higher than ours that is agreed upon by the majority of peers.
// This method may not return the absolute highest finalized, but the finalized epoch in which most peers can serve blocks.
// Ideally, all peers would be reporting the same finalized epoch but some may be behind due to their own latency, or because of
// their finalized epoch at the time we queried them.
// Returns the best finalized root, epoch number, and list of peers that are at or beyond that epoch.
func (p *Status) BestFinalized(maxPeers int, ourFinalizedEpoch uint64) ([]byte, uint64, []peer.ID) {
	connected := p.Connected()
	finalized := make(map[[32]byte]uint64)
	rootToEpoch := make(map[[32]byte]uint64)
	pidEpochs := make(map[peer.ID]uint64)
	potentialPIDs := make([]peer.ID, 0, len(connected))
	for _, pid := range connected {
		peerChainState, err := p.ChainState(pid)
		if err == nil && peerChainState != nil && peerChainState.FinalizedEpoch >= ourFinalizedEpoch {
			root := bytesutil.ToBytes32(peerChainState.FinalizedRoot)
			finalized[root]++
			rootToEpoch[root] = peerChainState.FinalizedEpoch
			pidEpochs[pid] = peerChainState.FinalizedEpoch
			potentialPIDs = append(potentialPIDs, pid)
		}
	}

	// Select the target epoch, which is the epoch most peers agree upon.
	var targetRoot [32]byte
	var mostVotes uint64
	for root, count := range finalized {
		if count > mostVotes {
			mostVotes = count
			targetRoot = root
		}
	}
	targetEpoch := rootToEpoch[targetRoot]

	// Sort PIDs by finalized epoch, in decreasing order.
	sort.Slice(potentialPIDs, func(i, j int) bool {
		return pidEpochs[potentialPIDs[i]] > pidEpochs[potentialPIDs[j]]
	})

	// Trim potential peers to those on or after target epoch.
	for i, pid := range potentialPIDs {
		if pidEpochs[pid] < targetEpoch {
			potentialPIDs = potentialPIDs[:i]
			break
		}
	}

	// Trim potential peers to at most maxPeers.
	if len(potentialPIDs) > maxPeers {
		potentialPIDs = potentialPIDs[:maxPeers]
	}

	return targetRoot[:], targetEpoch, potentialPIDs
}

// fetch is a helper function that fetches a peer status, possibly creating it.
func (p *Status) fetch(pid peer.ID) *peerStatus {
	if _, ok := p.status[pid]; !ok {
		p.status[pid] = &peerStatus{}
	}
	return p.status[pid]
}